1. Field of the Invention
The present invention relates to a system and method for determining the distance, speed and pace traveled by a person. More particularly, the invention concerns a system that is attached to the person, that processes and filters signals received from satellites to obtain kinematic measurements traveled by the person.
2. Description of the Related Art
For years runners have been attempting to precisely determine distance covered during their workout, their pace or current speed during their workouts. Although pedometers have been widely used for measuring distance, they have not proven to be sufficiently accurate. Also, pedometers are not able to directly indicate pace or speed. During races, rather than using pedometers, distance information is typically provided with distance markers placed along the course. While distance markers provide the runner with valuable distance information, they fail to directly indicate the runner's pace or speed. Average speed has typically been determined by measuring the elapsed time when the runner reaches a particular distance marker, and then manually dividing the distance by the elapsed time to calculate the average speed. Even the limited usefulness of distance markers is not available to runners on informal running courses, where distance markers are generally not available. Runners conducting interval work are forced to run on a measured track. When speed changes are required, current runners must calculate it mentally using time and distance.
In addition to runners, other athletes such as walkers, bicyclists, skiers, hikers, in-line skaters, swimmers, and triatheletes also frequently desire to determine the distance covered during their workouts, and their speed or variations of pace during their workouts. Additionally, distance and speed information is useful to health care professionals monitoring the exercise of their patients.
Navigation systems that calculate distance and speed information are widely known. For example, navigation systems that determine the latitude and longitude of ships and aircraft also commonly calculate the distance traveled and speed.
Global Positioning System (GPS) satellites have been widely used for navigational purposes to determine the latitude, longitude, and altitude of ships, aircraft and motor vehicles. Additionally, hand-held GPS receivers have been employed for mapping the latitude, longitude, and altitude of geographic locations on the earth.
GPS systems determine position by receiving signals from a sub-set of the 24 U.S. GPS satellites that are in operation. The signals transmitted by each satellite include a time code, which is synchronized with the time codes transmitted by the other satellites. The GPS system calculates an earth-centered-earth-fixed (ECEF) fix of a location where the signals are received, based on the time differences between the signals received from the satellites, and based upon the known locations of the satellites. ECEF is a 3-axis coordinate system with the origin located at the center of the earth. The satellites are not in geosynchronous orbits. The locations of the satellites are known because, prior to use the GPS system receives almanac and ephemeris data from the satellites. Almanac data is good for several weeks and is updated weekly. Ephemeris data is good for about 4 hours and is updated hourly. Almanac data consists of general information regarding all satellites in the constellation and ionospheric data for the determination of RF propagation delays. Almanacs are approximate orbital data parameters for all satellites. The typical ten-parameter almanacs describe the satellite orbits over extended periods of time of up to several months and a set for all satellites is sent to each satellite over a period of 12.5 minutes minimally. Signal acquisition time on receiver start-up can be significantly aided by the availability of current almanacs. The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency, (i.e. the frequency shift carried by the rate of change in range to a moving satellite), of each satellite in the constellation. Ephemeris data consists of detailed orbital information for the specific observed satellite. It can take up to 15 minutes to initialize a GPS system if the ephemeris data is down. The ephemeris data when down means that no ephemeris data is in system memory and/or the ephemeris data is obsolete.
With the exception of the P-coded GPS receivers used by the U.S. military, GPS receivers suffer from an error referred to as "Selective Availability." This error is purposefully introduced by the U.S. military into the signals transmitted by the GPS satellites, in order to prevent unfriendly forces form using the full potential of the system. The nature of the error is such that it will report a consistent deviation, typically about 100 meters, while the GPS receiver is processing signals from the same sub-set of satellites. For example, the error will consistently be 100 meters south-southeast. This type of error is significantly detrimental to navigational systems that attempt to determine location with reference to a global mapping system, such as the latitude, longitude, and altitude global mapping system of World Geodetic Survey 1984 (WGS-84). Due to this error, non U.S. military GPS systems and commercial systems which do not augment the GPS with "Differential" processing have not proven useful for applications in which 100 meters is a significant error, such as when attempting to record a runner's position versus time.
As a result of selective availability error, a related error is introduced into GPS systems when there is a change in the sub-set of satellites used to obtain a fix. This can occur when one or more satellites become obscured by terrain, vegetation, buildings, the user's body, or if one or more satellites sets over the horizon. This additional error manifests itself as a jump in the indicated position of the receiver.
In addition to the errors in determining position discussed above, typical handheld GPS receivers also suffer from limited memory storage capacity. Hand-held GPS receivers typically allow for the storage of approximately 500 way points. This memory is quickly used up if the system stores each successive fix along an athlete's path of travel.
Known GPS systems also have the shortcoming of being too large and heavy to be unobtrusively attached to the wrist, waist, or other convenient area of an athlete's body during a workout. The size of these units is partially the result of the relatively large space required for batteries, which is necessitated by the amount of power consumption.